February 10, Mr. Jeff Smith, Chairman Imperial Valley Water Authority E County Road 1000 N Easton, IL Dear Chairman Smith:

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1 February 1, 1 Mr. Jeff Smith, Chairman Imperial Valley Water Authority 8 E County Road 1 N Easton, IL Dear Chairman Smith: The Illinois State Water Survey (ISWS), under contract to the Imperial Valley Water Authority (IVWA), has operated a network of rain gauges in Mason and Tazewell Counties since August 199 and a network of groundwater observation wells since 199. The purpose of the rain gauge and groundwater observation well networks is to collect long-term data to determine the impact of groundwater withdrawals during dry periods and during the growing season, and the rate at which the aquifer recharges. This letter serves as the year end report for Year 17 which covers the time period from September 1, 8 through August 1, 9. Study Approach Observation Well Network The groundwater observation well network consists of thirteen wells, MTOW-1 through MTOW-1 (figure 1). MTOW-1, located near Snicarte, Illinois, is an inactive, large diameter, hand-dug domestic well that has been monitored by the ISWS since 198. This well is equipped with a Stevens, Type F water-level recorder that produces a continuous record of the groundwater level on a -day paper chart. The remaining observation wells are drilled wells between and inches in diameter. With the exception of MTOW- and MTOW-9, these wells are equipped with pressure transducers that electronically log the groundwater level data. In Year 1, a new well was drilled to replace MTOW-1. This new well, named Snicarte #, will eventually take the place of the original well (MTOW-1 or Snicarte #1) within the monitoring well network. The two wells are still being observed jointly so that their water level data can be correlated. The new well was originally equipped with the same type of Stevens recorder as the original well, but has since been converted to digital data collection. This digital data collection system is outfitted with telemetry which allows access to the data at the ISWS office via a cell phone modem. The final correlation between data from the two Snicarte wells will occur during Year 18. In accordance with our agreement, each well, with the exception of MTOW- and MTOW-9, is visited by ISWS personnel during the first few days of the month during irrigation season and approximately bi-monthly during the non-irrigated portion of the year. MTOW-1 is visited monthly because of its inclusion in the Shallow Groundwater Well Network of the ISWS Water and Atmospheric Resources Monitoring (WARM) Program. When the final correlation between Snicarte #1 and Snicarte # has taken place, Snicarte #1 will no longer be visited. Snicarte # will take its place within the monitoring well network and will be visited only a few times per year.

2 Figure 1. Configuration of the 1-site observation well and -site rain gauge networks, and location of the irrigation field site, Imperial Valley, 8-9. Groundwater levels for each well for the period of record (September 1, 8-August 1, 9) are presented in Appendix A. For MTOW-1, -, and -9 their entire period of record is shown because they do not have digital recorders. In future reports, MTOW-1 will follow the yearly format as the well is now equipped with a digital recorder which allows for daily data collection. Each hydrograph also contains the daily precipitation for the nearest rain gauge. For observation wells located between several rain gauges, an average of the surrounding rain gauge data is presented. Groundwater level data are presented as groundwater elevation above mean sea level. Rain Gauge Network A -site rain gauge network (Figure 1) with approximately miles between gauges was established in late August 199. The network was reduced to sites in September 199. The rain gauge network is maintained by a Mason County resident hired to visit each site monthly. During

3 these visits the charts are changed, data downloaded and other routine services performed. ISWS personnel visit the rain gauge network to perform major maintenance and repairs as needed. Data reduction activities during Year Seventeen of network operation are similar to those performed during the previous sixteen years. Each month rainfall amounts are totaled from 1- minute digital data and are placed into an array of values for the gauges. This data array is used to check for spatial and temporal consistency between gauges, and to divide the data into storm periods. If the digital data are missing, hourly rainfall amounts from the analog (paper) charts are used. In the rare event that data from both a data logger and the corresponding chart are missing, the hourly amounts are estimated based on an interpolation of values from the nearest surrounding gauges. Irrigation Since 199, the IVWA has estimated irrigation pumpage from wells in the Imperial Valley based on electric power consumption. Menard Electric Cooperative provides the IVWA with electric power consumption data for the irrigation services they provide during the growing season (June-September). The pumpage estimate assumes that application rates for the irrigation wells with electric pumps in Menard Electric Cooperative also are representative of other utilities and other energy sources. Past estimates were based on the assumption that percent of the irrigation wells were served by Menard Electric Cooperative in , percent in In, the U.S. Geological Survey (USGS) updated the formula used to calculate pumpage by closely measuring the pumping rate at 77 irrigation systems serviced by Menard Electric. The updated formula provides estimates that are appreciably lower than the previous formula, by approximately percent. Therefore, irrigation withdrawals for the years 1997 to the present were recalculated using the new formula, replacing earlier published estimates (reports through Year 11, September 1, August 1,, use the original formula). Data Analysis The Year Seventeen dataset was used to produce summaries for all storm data for each station and the network; monthly, seasonal, and annual rainfall totals; analysis of the rainfall and river stages and groundwater level fluctuations; the data obtained from the long-term monitoring well network; the database showing the individual storms in the Imperial Valley region; and an updated version of the irrigation pumpage data. Rain Gauge Network The Year Seventeen network precipitation of 9. inches was above average, 1. inches above the previous 1-year s average of.79 inches. It was the second wettest year in the 17 years of network operation. The spring, summer and autumn seasons in Year Seventeen were above average in seasonal total precipitation. Table 1 gives the monthly precipitation totals for each rain gauge within the network during Year 17.

4 Table 1. Monthly Precipitation Amounts (inches), September 8- August 9 Month Station Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Total Avg Figure presents the 17-year network average, excluding sites 1, 19, and 1 during the period 1997-, and Figure presents the annual precipitation pattern for Year Seventeen. During Year Seventeen, annual gauge totals varied from.8 inches at site to.9 inches at site number (Figure ). Eight-inch gradients between gauges in annual precipitation are not unusual during any given year, as long as they are not replicated at the same gauges year after year, and are somewhat supported by surrounding gauges. September and December of 8 and February, March, April, June, July and August of 9 all received above average precipitation with September 8 being the wettest month of the year. All other months of the year received below average precipitation (Figures -9). September 8 accounted for 1. inches of precipitation or 1 percent of the 1-year average annual precipitation, and 1 percent of Year Seventeen total network precipitation. The network received.1 inches less precipitation than in the wettest year ( ) and. inches more than in the driest year ( ). Storm Events The number of network precipitation periods was determined for the 17-year period. Mean monthly, seasonal, and annual number of these precipitation events are presented for 8-9 (Table ). The monthly, seasonal, and annual number of precipitation events averaged over the period also is presented (Table ). A network storm period is defined as a precipitation event separated from proceeding and succeeding events at all network stations by at least three hours.

5 Figure. Network average annual precipitation (inches) for September August 9 Figure. Total precipitation (inches) for September 8- August 9

6 Figure. Precipitation (inches) for September 8 and October 8

7 Figure. Precipitation (inches) for November 8 and December 8 7

8 Figure. Precipitation (inches) for January 9 and February 9 8

9 Figure 7. Precipitation (inches) for March 9 and April 9 9

10 Figure 8. Precipitation (inches) for May 9 and June 9 1

11 Figure 9. Precipitation (inches) for July 9 and August 9 11

12 Table. Comparison of Total Precipitation (inches), Number of Precipitation Events, and Average Precipitation per Event for Each Month and Season, and yr average 8-9 average Period Precipitation Events Inches/event Precipitation Events Inches/event Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Fall Winter Spring Summer Annual A total of storm periods occurred during the 17-year observation period, with 1 in 8-9, resulting in an average of 11 storm events per year. Fewer events than average occurred in October 8 and January 9. A greater number of events than average occurred in November 8 and April, May, and June of 9. The spring had a larger number of events than average. Fall 8, Spring 9 and Summer 9 received more precipitation than average and the events during these seasons were more intense than average. The plot of the network average monthly precipitation time series (Figure 1) shows the monthly variation of precipitation. From February through August 8, a very dry period of record, only six months had precipitation of greater than. inches. During 8-9, seven months received inches of precipitation or greater. The storm recurrence frequency is the statistical probability of the recurrence of a storm with the reported precipitation (i.e., a 1-year storm would be expected to occur on average only once every 1 years at a given station, or have a 1 percent chance of occurring in any given year). The recurrence frequencies computed here are for each gauge and are based upon the total storm duration for the gauges with precipitation. In the first 1-years of network operation, 71 of the 198 storm events produced maximum precipitation at one or more gauges with a recurrence frequency greater than one year: -yr (1 storm), 1-year ( storms), year (9 storms), year ( storms), and greater than 1-year but less 1

13 Total Monthly Precipitation (inches) Figure 1. Network average monthly precipitation (inches), September August 9 than -year ( storms). The -year storm occurred on 1 September 199, and the 1-year storms on 1 May 199, 8 May 199, 19 July 1997, and March 7. In Year 17, nine of the 1 network storm periods exceeded the one-year or greater recurrence frequency. One event exceeded the 1-year recurrence frequency and occurred from 11-1 September 8. The network-averaged precipitation totaled.1 inches for this event. One event exceeded the -year recurrence frequency and occurred from 8-9 July 9. Groundwater Levels The long-term hydrograph at MTOW-1 (Snicarte) in Figure 11 provides a reference for comparison with the shorter records of the other network wells. The ISWS has recorded water levels in this well since 198. Annual fluctuations from less than a foot to more than feet have been observed. Based on the data we have available, these annual fluctuations often appear to be superimposed on longer term trends, perhaps 1 years or more. For the 1-year record, both the record low and high have been observed within the last years. A detailed look at water levels since 199 is shown in Figure 1. During and shortly after the drought years of 1988 and 1989, the water level fell to. feet below land surface from September 1989 until April 199, the only time in its -year history that the well went dry, until it did so again in and 7. During the 199 flood, groundwater levels rose almost 1 feet and peaked at approximately feet in September

14 8 Great Flood of 199 aa Depth to water, feet Drought Well Dry Year Figure 11. Groundwater levels at the Snicarte well, MTOW-1, Flood Depth to water (feet) Drought Well Dry Year Figure 1. Groundwater levels at the Snicarte well, MTOW-1,

15 The dramatic drop in shows how significantly a major drought can impact the aquifer. Though we don t have irrigation data for 1988, based on data from the other parts of the state (Cravens, 1989) it is likely that irrigation in 1988 was one of the highest amounts of any year. This is because summer precipitation was so low and summer temperatures were so high in Similarly, the irrigation amounts in (7 billion gallons) were 1 percent of average since 199 and we see similar dramatic declines in water levels. Again in, temperatures were high and summer precipitation was low. Conversely, 8-9 was a relative wet year with low irrigation withdrawals and ascension in water levels was observed. It is likely that local summer precipitation in future years will cause similar fluctuations in aquifer levels. Over the course of the last few years, the study area has received below average rainfall. These below average precipitation totals in combination with irrigation withdrawals have affected the groundwater elevations in the study area. This began in -, when rainfall amounts fell below average and continued to fall overall until this year. Year Seventeen precipitation has elevated groundwater levels to the point of near record highs since the observation well network was established in 199. Previous reports have shown hydrographs indicating recharge events in the aquifer occurring within a few days after a rainfall event. In other words, recharge occurs on a scale of days after a precipitation event, and so historical monthly measurements missed many such events. Based on these results, the IVWA purchased ten In-Situ data loggers that were installed in wells between December, and August. The hydrographs for these ten loggers can be seen in Appendix A. In Year Seventeen, a rainfall event during September at the Easton Well (MTOW-) is very evident as recharge as shown in Figure 1. The four separate rainfall events during early September produced nearly five feet of recharge within approximately seven days. Other regions of the aquifer show a similar but less pronounced response. Figures 1 and 1 show the same precipitation event at the Hahn Farm (MTOW-1) and San Jose wells (MTOW-1) producing over 1 foot of recharge within seven days. And while the hydrographs showing the recharge following precipitation for the Hahn Farm and San Jose wells are not as dramatic as at Easton, in general, the information they provide are just as vital. The recent addition of continuous water level measurements has already led to an increased understanding of the relationship between rainfall, irrigation, water levels, and recharge. Along with Easton, Hahn Farm and San Jose, the hydrographs showing continuous water levels and daily rain gauge data for Year 17 are available for the remaining monitoring wells in Appendix A. Groundwater levels in the Pekin (MTOW-) and Havana-IDOT (MTOW-9) wells, because of their proximity to the Illinois River, have been found to fluctuate largely in response to river stage. Since these two monitoring wells are so strongly influenced by the Illinois River, the wells are not outfitted with pressure transducers and are measured infrequently. The hydrographs for these two wells (MTOW- and MTOW-9) are located in Appendix A. 1

16 Groundwater Elevation (feet above MSL) MTOW- and Average of Rain Gauges 1, and 1 September 1, 8 - September 1, 8 Land Surface Elevation 9.8 Digital Measurement Observed Measurement Averaged Daily Precipitation at Rain Gauges 1, and 1 1 Precipitation (inches) 97 9/1/8 9/7/8 9/1/8 9/19/8 Figure 1. Groundwater elevations at the Easton well, MTOW-, September 1, 8-September 1, 8 9 Groundwater Elevation (feet above MSL) MTOW-1 and Average of Rain Gauges 1 and September 1, 8 - September 1, 8 Land Surface Elevation.7 feet above MSL Digital Measurement Observed Measurement Averaged Daily Precipitation at Rain Gauges 1 and 1 Precipitation (inches) 8 9/1/8 9/7/8 9/1/8 9/19/8 Figure 1. Groundwater elevations at the Hahn Farm, MTOW-1, September 1, 8-September 1, 8 1

17 8 Groundwater Elevation (feet above MSL) MTOW-1 and Rain Gauge 1 September 1, 8 - September 1, 8 Land Surface Elevation.7 feet above MSL Digital Measurement Observed Measurement Daily Precipitation at Rain Gauge 1 1 Precipitation (inches) 8 9/1/8 9/7/8 9/1/8 9/19/8 Figure 1. Groundwater elevations at the San Jose wells, MTOW-1, September 1, 8-September 1, 8 Irrigation Water Use For Year Seventeen, the higher than normal precipitation in the spring and summer of 9 affected irrigation practices significantly. Irrigation in June was the second lowest June for the length of the study, July the third lowest, August the fourth lowest and September the lowest on record. The total irrigation pumpage in 9 was approximately billion gallons (bg), which is the lowest irrigation amount for the observation period, below the approximately bg pumped in 1998 and, when there were several hundred fewer irrigation systems in use. The monthly and seasonal estimates of irrigation withdrawals are shown in Table. These data were calculated for the Imperial Valley by previously described methods. Total annual irrigation withdrawals, from highest to lowest, are as follows:, 7, 199, ; 1 and (equal); ; ; 1999; 1997 and 199 (equal); 8; 1998 and (equal); and 9. Typically, irrigation withdrawals are greatest in July and August, with September and June withdrawals being much less. Though more irrigation systems are added each year, suggesting that irrigation pumpage should keep increasing, it is clearly apparent that the timing and amount of rainfall received during the irrigation season (rather than throughout the whole year) are primary factors affecting the amount of irrigation. 17

18 Table. Estimated Monthly Irrigation Withdrawals (billion gallons), Number of Irrigation Systems, Withdrawal per System and Withdrawal Rank Year June July August September Total # Systems BG/system Rank Average Note: Total annual withdrawal may differ from sum of monthly withdrawals due to rounding error. Also, data regarding the number of systems in are unavailable. The estimated monthly irrigation pumpage is displayed graphically in Figure 17 along with average monthly network precipitation. These pumpage values show a tendency for lower irrigation amounts during times of increasing precipitation and vice versa, but also show that irrigation is dependent on the timing of precipitation. For example, only bg were pumped in (Year Eight), even though Year Eight showed a deficit of 9. inches (Table ). This was because significant precipitation fell during the summer of, reducing the need for irrigation. Similarily, Year Fifteen (summer 7) was the ninth driest of network operation, but ranked number for irrigation pumpage. The influence of the abundant rainfall during Year 17, 1.8 inches of precipitation above average, is evident in both the decreased amount of water withdrawn for irrigation and in higher groundwater levels throughout the study area. Table also shows that for the of the last 7 years and for 11 of the last 1 years, rainfall has been below the -year (1971-) historical average of.7 inches (average of Havana and Mason City). 18

19 Estimated monthly irrigation pumpage (June-September) Monthly precipiation Totals 1 Estimated monthly irrigation pumpage, billion gallons Monthly Precipitation (inches) Figure 17. Estimated irrigation pumpage and average monthly precipitation, Imperial Valley 19

20 Table. Average Annual Precipitation, Annual Precipitation Surplus, Running Surplus, and Ranked Annual Precipitation and Irrigation, Imperial Valley Network September-August Network average Annual Running Rank period precipitation (in.) surplus (in.) surplus (in.) Precip. Irrigation yr average 7.8 (Havana) yr average.7 (Mason City) yr average.7 (average of Mason City and Havana used to determine surplus) Note: Site 1 was excluded from network average computations from through 1-. Findings In summary, for Year Seventeen of the rain gauge network operation (September 8- August 9), the network received an average of 9. inches of precipitation, 1. inches more than the network previous 1-year average precipitation of.79 inches and 1.8 inches more than the -year average for the study area. The timing and amount of rainfall was such that the need for irrigation in the summer of 9 was greatly reduced. This reduced pumping of the aquifer for irrigation, along with above average precipitation, provided an opportunity to recharge the aquifer system significantly. In fact, groundwater levels within the aquifer are dramatically higher at the end of Year 17 than at the beginning. The ten pressure transducers installed in the observation wells give us a much better picture of how rainfall, depth to water, and distance to a nearby stream influence recharge and water level changes in the aquifer. This past year, with several significant storm events, we were able to record and see measurable response in the aquifer that had not been seen the past several years at some locations. We look forward to gathering additional supporting data that will allow us to better predict water level changes in the future.

21 If you have any questions or comments, please contact Kevin Rennels, Steve Wilson or Nancy Westcott. Sincerely, Kevin L. Rennels Steven D. Wilson Nancy Westcott Associate Supportive Scientist Groundwater Hydrologist Research Meteorologist Illinois State Water Survey Illinois State Water Survey Illinois State Water Survey Phone: (17) -8 Phone: (17) -9 Phone: (17) -88 c: Dorland Smith Wayne Deppert Don Osborn, Jr. Morris Bell 1

22 Appendix A. Hydrographs, Imperial Valley Observation Well Network Depth to water at MTOW-1, feet below land surface Observed depth to water in MTOW-1 Total monthly precipitation at Gauge Figure A-1. Groundwater depth and precipitation for MTOW Total monthly precipitation at Gauge, inches Groundwater Elevation (feet above MSL) MTOW- and Average of Rain Gauges 1, and 1 September 1, 8 - August 1, 9 Land Surface Elevation 9.8 Digital Measurement Observed Measurement Averaged Daily Precipitation at Rain Gauges 1, and 1 1 Precipitation (inches) 97 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-. Groundwater depth and precipitation for MTOW-

23 8 Groundwater Elevation (feet above MSL) MTOW- and Rain Gauge 18 September 1, 8 - August 1, 9 Land Surface Elevation 9. feet above MSL Digital Measurement Observed Measurement Daily Precipitation at Rain Gauge 18 1 Precipitation (inches) 7 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-. Groundwater depth and precipitation for MTOW- Groundwater Elevation (feet above MSL) MTOW- and Rain Gauge September 1, 8 - August 1, 9 Land Surface Elevation 87.1 feet above MSL Digital Measurement Observed Measurement Daily Precipitation at Rain Gauge 1 Precipitation (inches) 9 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-. Groundwater elevation and precipitation for MTOW-

24 Depth to water in MTOW-, feet below land surface Observed depth to water in MTOW- Ave. of total monthly precipitation at Gauges and Figure A-. Groundwater depth and precipitation for MTOW Average of total monthly precipitation at Gauges and 8 Groundwater Elevation (feet above MSL) MTOW- and Average of Rain Gauges 9,1 and 1 September 1, 8 - August 1, 9 Land Surface Elevation 9. feet above MSL Digital Measurement Observed Measurement Average Daily Precipitation at Rain Gauges 9,1 and 1 1 Precipitation (inches) 7 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-. Groundwater elevation and precipitation for MTOW-

25 8 Groundwater Elevation (feet above MSL) MTOW-7 and Average of Rain Gauges 1, and1 September 1, 8 - August 1, 9 Land Surface Elevation 9.7 feet above MSL Digital Measurement Observed Measurement Average of Daily Precipitation at Rain Gages 1, and 1 1 Precipitation (inches) 7 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-7. Groundwater elevation and precipitation for MTOW-7 1 Groundwater Elevation (feet above MSL) MTOW-8 and Rain Gauge 7 September 1, 8 - August 1, 9 Land Surface Elevation 1.99 feet above MSL Digital Measurement Observed Measurement Daily Precipitation at Rain Gauge 7 1 Precipitation (inches) 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-8. Groundwater elevation and precipitation for MTOW-8

26 Depth to water in MTOW-9, feet below land surface 1 1 Observed depth to water in MTOW-9 Ave. of total monthly precipitation at Gauges 8 & Figure A-9. Groundwater depth and precipitation for MTOW Average of total monthly precipitation at Gauges 8 & 1 Groundwater Elevation (feet above MSL) 1 9 MTOW-1 and Rain Gauge 1 September 1, 8 - August 1, 9 Land Surface Elevation.7 feet above MSL Digital Measurement Observed Measurement Daily Precipitation at Rain Gauge 1 1 Precipitation (inches) 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-1. Groundwater depth and precipitation for MTOW-1

27 Groundwater Elevation (feet above MSL) 8 MTOW-11 and Average of Rain Gauges and September 1, 8 - August 1, 9 Land Surface Elevation 1. feet above MSL Digital Measurement Observed Measurement Average Daily Precipitation at Rain Gauges and 1 Precipitation (inches) 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-11. Groundwater elevation and precipitation for MTOW-11 9 Groundwater Elevation (feet above MSL) MTOW-1 and Average of Rain Gauges 1 and September 1, 8 - August 1, 9 Land Surface Elevation.7 feet above MSL Digital Measurement Observed Measurement Averaged Daily Precipitation at Rain Gauges 1 and 1 Precipitation (inches) 8 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-1. Groundwater elevation and precipitation for MTOW-1 7

28 7 Groundwater Elevation (feet above MSL) 8 MTOW-1 and Rain Gauges September 1, 8 - August 1, 9 Land Surface Elevation 1.9 feet above MSL Digital Measurement Observed Measurement Daily Precipitation at Rain Gauges 1 Precipitation (inches) 9/1/8 1/1/8 11/1/8 1/1/8 1/1/9 /1/9 /1/9 /1/9 /1/9 /1/9 7/1/9 8/1/9 9/1/9 Figure A-1. Groundwater depth and precipitation for MTOW-1 8